74 A NATURALIST IN THE GREAT LAKES REGION 



bubbles of gas contained in the molten lava. The holes formed 

 by the included bubbles of gas may later be filled by material 

 deposited by percolating water. The grains of such deposited 

 minerals are naturally rounded, and the rock so altered is known 

 as an amygdaloid. Molten material, on the other hand, that 

 cools off slowly way down below the surface makes coarse-grained 

 rock, the crystals being large. Such rocks are called plu tonic, 

 in distinction to the volcanic. If one mineral is present in large 

 crystals while the others are minutely crystallized or more or 

 less glassy, the rock is called a porphyry. Plutonic rocks are 

 dense, since formed below the surface under great pressure. 

 Naturally there will be all intergrades between the plutonic and 

 the volcanic rocks, since the cooling lava may be in any one of 

 many situations from the deep-seated reservoir to the surface 

 flow. 



The molten material differs greatly in chemical composition 

 in different regions of the earth and even two successive lava 

 flows from the same volcanic crater may be quite unlike. If the 

 lava contains much silica it is an acid lava and gives rise to rocks 

 with many silicates and much free silica or quartz. If, on the 

 other hand, it contains strong bases like calcium, magnesium, 

 and iron, it is a basic lava and the resulting rocks will contain 

 little free silica or quartz, though they will contain silicates of 

 the basic elements mentioned in the form of such minerals as 

 plagioclase, hornblende, augite, etc. The following scheme with 

 the brief characterizations that follow will aid in the determina- 

 tion of the igneous rocks of the Chicago area. Nothing like a 

 complete key or complete descriptions can be given here, and 

 the interested student will secure some good lithology and 

 study it in connection with the specimens of typical rocks found 

 in the museums. This scheme will help the observer to make a 

 start on interpreting the past history of a rock from its present 

 structure. Five rock groups are given in the order of their 

 increasing basidity so that, from left to right in the series, quartz 

 is becoming less abundant, the rocks are darkening in color and 



